The present invention relates to a projection lens system and particularly to a projection lens system with a wide field angle which provides a bright image having an excellent focus performance even in the marginal area, uses an inexpensive glass material, and has a short projection distance and a projection image display apparatus using the system which is excellent in cost performance.
Recently, a television set as an image display apparatus for home use is proceeding to a larger screen size as the wide aspect ratio increases. As an image display apparatus for home use, there are two types available such as a direct view type using a cathode ray tube and a so-called projection type for enlarging and projecting an image on a miniature projection tube whose screen size is about 7 inch of diagonal on the screen by a projection lens system. However, due to restrictions to compactness and weight of a TV set, for a screen size of more than about 37 inch of diagonal, a projection image display apparatus is mainly used.
At first, this projection image display apparatus was inferior to the direct view type in screen brightness and focus performance. However, recently, the performance of each of the components such as the projection lens system, screen, and projection tube is improved, so that both the screen brightness and focus performance are approaching those of the direct view type. In the performance improvement process of the projection image display apparatus, various arts have been developed in the projection lens system which is a key device. Firstly, at the first step of development, to obtain screen brightness equivalent to that of the direct view type or higher, as disclosed in U.S. Pat. No. 4,682,862, reduction of the F-number has been tried by using many plastic aspherical lens elements.
Next, at the second step, a projection lens system for realizing improvement of screen brightness and improvement of focus performance at the same time has been developed. With respect to this projection lens system, as disclosed in Japanese Patent Application Laid-Open No. 3-137610, there is an example using a plastic aspherical lens and a doublet glass lens. As a result, in the current projection TV set, a projection lens system having an F-number of about f/1.1 is used and both brightness and focus performance are improved on the whole screen.
At the third step, a projection lens system with a wide field angle by which a compact set dimension can be realized on account of the short projection distance has been developed mainly. A reference describing an actual art for realizing a projection lens system with a wide field angle without reducing the brightness and focus performance in the marginal area and an actual projection lens system is disclosed in Japanese Patent Application Laid-Open No. 4-5608. Hereinafter, the art disclosed in this patent is referred to as a first prior art.
In this first prior art, by combining plastic aspherical lens elements and glass lens elements effectively in a projection lens system of six lens groups, the aforementioned problem is solved. Furthermore, the projection lens system is structured so that almost all the positive refractive power of the projection lens system is shared by the glass lenses and the plastic aspherical lens elements have little refractive power, so that the peculiar drift of the focus performance due to a temperature change is reduced even if the plastic aspherical lens elements are used is reduced.
In this first prior art, the profile of the fluorescent face of projection tube has a curvature so that it is convex on the electron gun side. As a result, the projection lens system is structured so that the normal of the fluorescent face in the marginal area is in the direction of the entrance pupil of the projection lens system and can fetch more light fluxes in comparison with the case using a flat fluorescent face. Therefore, even if the field angle is widened, a relative illuminance of a level which is almost no problem practically can be obtained in the marginal area.
The curvature of field is corrected by the lens element of the sixth lens group (hereinafter referred to as sixth lens). However, if the fluorescent face of projection tube has a curvature so that it is convex on the electron gun side, the generation amount of curvature of field is reduced and the focus performance in the marginal area is improved.
Furthermore, a projection lens system with a wide field angle which realizes a more excellent focus performance without reducing the brightness in the marginal area and an actual art for realizing it are disclosed in U.S. Pat. No. 5,272,540. Hereinafter, the art is referred to as a second prior art.
In the second prior art, a projection lens system having a constitution of five groups by six elements is disclosed and the profile of the fluorescent face of projection tube which is an object is an aspherical profile which is convex on the electron gun side. And has when the curvature of the profile in the marginal area is smaller than that in the neighborhood of the optical axis. By doing this, highly precise correction of the curvature of field and astigmatism are compatible with each other and the satisfactory focus performance and the light amount which is practically sufficient are reserved in the marginal area of screen.
In this projection lens system, the lens element of the third lens group (hereinafter referred to as third lens) sharing almost all refractive power of the overall lens system has a constitution that a concave lens of large dispersion glass and a convex lens of small dispersion glass are stuck together, and the chromatic aberration is corrected, and the large aperture (the F-number is 0.96) and the high focus performance are compatible with each other. Furthermore, it is structured that combination of the lens element of the first lens group (hereinafter referred to as first lens) and the lens element of the second lens group (hereinafter referred to as second lens) offsets the lowering of the focus performance generated by deformation and expansion of each lens element due to temperature change and humidity change which is an intrinsic problem when plastic lens elements are used.
On the other hand, in a conventional projection lens system, as a lens barrel for assembling each lens element with high precision, a lens barrel having the constitution disclosed in, for example, Japanese Utility Model Application Laid-Open No. 2-51478 is often used. The lens barrel of the prior art has an outer barrel and an inner barrel which is installed inside the outer barrel and can slide in the direction of optical axis of the lens without axial shift. The inner barrel has a constitution that it can be divided into two parts longitudinally in the direction of diameter of the lens along the optical axis of the lens and it has slits for holding a plurality of lens elements at predetermined intervals with high precision on its inner surface.
In the aforementioned projection lens system having a constitution of six lens groups of the first prior art, there are several problems to be solved.
The first problem is a problem caused by the lens constitution. In the aforementioned projection lens system, the third lens having negative refractive power is arranged on the screen side of the lens element of the fourth lens group (hereinafter referred to as fourth lens) sharing almost all the positive refractive power of the overall lens system. The spherical aberration and coma aberration are corrected by the third lens.
Therefore, the location of the entrance pupil of the overall lens system moves to the screen side from the center of the fourth lens. As a result, if an attempt is made to realize a wider field angle (reduction of the projection distance) in the aforementioned lens constitution, correction of the distortion and astigmatism becomes difficult.
Next, the second problem is a point that if an attempt is made to reduce the F-number or (increase the aperture ratio) of the projection lens system having this lens constitution and obtain a sufficient marginal light amount ratio, the apertures of the first, second, and third lenses become larger and the production cost increases.
The share of correction of each lens group in aberration correction of the aforementioned projection lens system is shown below.
The first lens is a spherical lens element of a meniscus profile having positive refractive power and corrects spherical aberration and coma aberration.
The second lens is a plastic aspherical lens element of a meniscus profile having weak positive refractive power and corrects spherical aberration and coma aberration.
The third lens is a spherical lens element having a weak divergent action and corrects spherical aberration and coma aberration.
The fourth lens is a convex--convex glass spherical lens element having a strong convergent action.
Furthermore, the lens element of the fifth lens group (hereinafter referred to as fifth lens) is a plastic aspherical lens element of a meniscus profile having weak positive refractive power and corrects astigmatism, distortion, and coma aberration.
The sixth lens has a constitution that it has a concave surface facing the screen side, has negative refractive power accompanied by a liquid coolant (A), and corrects curvature of field.
Among them, the second lens and fifth lens are a plastic aspherical lens element and have a meniscus profile having weak positive refractive power respectively. This projection lens system of prior art has a constitution that each plastic lens element has little refractive power and the peculiar shift of the focus performance due to a temperature change when the plastic aspherical lens element is used is reduced.
There is a third problem imposed that as mentioned above, in the projection lens system using the first prior art, the applicable lens profile of the plastic aspherical lens is limited to a specific profile and that the aberration correction cannot be attained sufficiently.
A fourth problem is also imposed that since four glass lens elements are included, the cost is increased.
Furthermore, the aspherical surface amount of the fifth lens is little, and the sixth lens is a glass lens element, whereof the lens surface on the screen side is a spherical surface, so that correction of astigmatism and correction of curvature of field are not compatible with each other.
Therefore, a fifth problem arises that correction of astigmatism in the marginal area is difficult.
In the first prior art, it is a subject (of the design) to solve these problems.
A problem of the projection lens system having a constitution of five groups by six elements to be solved in the second prior art is reduction in cost.
The two factors for an increase in the cost of the projection lens system are described below.
The first factor for an increase in cost is the profile of fluorescent face of the projection tube. The main profile of fluorescent face of the projection tube is a spherical fluorescent face at present. When this projection lens system is applied, it is necessary to make the profile of fluorescent face aspherical and the projection tube is to be produced under a special specification, so that it is a factor for an increase in the cost of the set.
The second factor for an increase in cost is that it is essential to use a doublet lens comprising a large dispersion concave lens with a large diameter and a small dispersion convex lens with a large diameter which are stuck together for the third lens so as to realize a large aperture ratio (the F-number is 0.96) in this projection lens system and correct chromatic aberration satisfactorily.
Generally, the price of optical glass increases as the refractive index increases and as the dispersion decreases. In the second prior art, the optical glass used as a third lens of the projection lens system described in Embodiment 1 includes large dispersion glass of SF11 and small dispersion glass of SK16. The prices of these optical glass materials are more than 2 times as expensive as the price of SK5 which is a typical one of optical glass used in the projection lens system such that the price is 2.3 for SF11 and is 2.1 for SK16 (those glass names are abbreviations of Schott, Ltd. and often used in this field).
On the other hand, a problem when the aforementioned conventional lens barrel is used in the projection lens system is that the air temperature in the sealed space inside the lens barrel and the temperature of the lens elements rise, and the heated lens elements expands and deforms, and the focus performance of the projection lens system is lowered extremely.
As a result, it is a subject (of the design) to suppress rising of the air temperature in the sealed space inside the lens barrel and the temperature of the lens elements and to prevent the lens elements from expansion and deformation even if the heat generated from an image generating source is high.